Since a photographic process using silver halide is excellent in photographic properties such as sensitivity and gradation control as compared to other photographic processes such as electrophotography and diazo photographic process, the silver halide photographic process has hitherto been most widely used. Recently, a technique capable of more easily and rapidly obtaining images has been developed by changing an image-forming process of a silver halide photographic material from a conventional wet process such as a process which uses a liquid developer to a dry process such as a developing process which uses heating.
Heat developable photographic materials are known in this art and the heat developable materials and image-forming processes using these heat developable materials are described in, for example, "Shashin Kogaku no Kiso (The Basis of Photographic Engineering)", pages 553-555, published by Corona K. K. in 1979; "Eizo Jooho (Image Information)", page 40, published in April, 1978; "Nebletts Handbook of Photography and Reprography", 7th Ed., pages 32-33, (Van Nostrand Reinhold Company); U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020, 3,457,075; U.K. Patent Nos. 1,131,108 and 1,167,7;77; and "Research Disclosure" (RD-17029), pages 9-15, June 1978.
Various processes have been proposed for obtaining dye images by a dry system. For example, for forming color images by a combination of the oxidation product of a developing agent and a coupler, there are proposed a combination of a p-phenylenediamine reducing agent and a phenolic or active methylene coupler in U.S. Pat. No. 3,531,286; p-aminophenol series reducing agents in U.S. Pat. No. 3,761,270; p-aminophenol series reducing agents in Belgian Patent No. 802,519 and a sulphonamidophenol series in "Research Disclosure", pages 31-32, September 1975; and a combination of a sulfonamidophenol series reducing agent and a 4-equivalent coupler in U.S. Pat. No. 4,021,240.
However, in these processes there is a fault that color images become turbid since images of reducing silver and color images are simultaneously formed at light exposed areas after heat development. For overcoming the fault, there are proposed a process of removing the silver images by liquid processing and a process of transferring the dyes only to another layer, for example, a sheet having an image-receiving layer. However, there remains a fault that it is not easy to discriminate a dye from the reaction mixture and transfer the dye by itself.
Also, a process of introducing a nitrogen-containing heterocyclic ring group into a dye, forming a silver salt, and liberating the dye by heat development is described in "Research Disclosure", pages 54-58, May 1978. However, in the process it is difficult to control the liberation of the dye at non-exposed areas, whereby a clear image cannot be obtained and hence the foregoing process is unsuitable for general use.
Furthermore, a process of forming color images by utilizing leuco dyes is described in, for example, U.S. Pat. Nos. 3,985,565 and 4,022,617. However, in the process it is difficult to stably incorporate leuco dyes in photographic materials, that is, the photographic materials containing leuco dyes are gradually colored when they are preserved.
Also, for forming a positive color image by a heat sensitive silver dye bleaching process, there are described useful dye bleaching processes in "Research Disclosure", pages 30-32, April 1976; ibid., pages 14-15, December 1976; U.S. Pat. No. 4,235,957, etc.
However, these processes have such faults that additional steps and material are required for accelerating the bleaching of dye, for example, it is required to superpose an activating agent sheet on the light-sensitive material and heat the assembly, and also color images obtained are gradually bleached by reduction with free silver, etc., existing in the light-sensitive material during the preservation of the images for a long period of time.